Intracluster medium cooling, AGN feedback, and brightest cluster galaxy properties of galaxy groups^⋆

Five properties where groups differ from clusters

¹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: bharadwaj@astro.uni-bonn.de
² Center of Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA
³ Department of Physics, Durham University, South Road, Durham DH1 3LE, UK

Received: 16 September 2013
Accepted: 3 February 2014

Abstract

Aims. We aim to investigate cool-core and non-cool-core properties of galaxy groups through X-ray data and compare them to the AGN radio output to understand the network of intracluster medium (ICM) cooling and feedback by supermassive black holes. We also aim to investigate the brightest cluster galaxies (BCGs) to see how they are affected by cooling and heating processes, and compare the properties of groups to those of clusters.

Methods. Using Chandra data for a sample of 26 galaxy groups, we constrained the central cooling times (CCTs) of the ICM and classified the groups as strong cool-core (SCC), weak cool-core (WCC), and non-cool-core (NCC) based on their CCTs. The total radio luminosity of the BCG was obtained using radio catalogue data and/or literature, which in turn was compared to the cooling time of the ICM to understand the link between gas cooling and radio output. We determined K-band luminosities of the BCG with 2MASS data, and used a scaling relation to constrain the masses of the supermassive black holes, which were then compared to the radio output. We also tested for correlations between the BCG luminosity and the overall X-ray luminosity and mass of the group. The results obtained for the group sample were also compared to previous results for clusters.

Results. The observed cool-core/non-cool-core fractions for groups are comparable to those of clusters. However, notable differences are seen: 1) for clusters, all SCCs have a central temperature drop, but for groups this is not the case as some have centrally rising temperature profiles despite very short cooling times; 2) while for the cluster sample, all SCC clusters have a central radio source as opposed to only 45% of the NCCs, for the group sample, all NCC groups have a central radio source as opposed to 77% of the SCC groups; 3) for clusters, there are indications of an anticorrelation trend between radio luminosity and CCT. However, for groups this trend is absent; 4) the indication of a trend of radio luminosity with black hole mass observed in SCC clusters is absent for groups; and 5) similarly, the strong correlation observed between the BCG luminosity and the cluster X-ray luminosity/cluster mass weakens significantly for groups.

Conclusions. We conclude that there are important differences between clusters and groups within the ICM cooling/AGN feedback paradigm and speculate that more gas is fueling star formation in groups than in clusters where much of the gas is thought to feed the central AGN.